Process for obtaining compensation quantity to compensate the nonuniformity of a surface wave convolver

ABSTRACT

A process obtains a compression quantity to compensate the nonuniformity of a surface wave convolver. A surface wave convolver includes an elongate piezoelectric crystal, to the ends of which interdigital transducers are fitted to convert electrical input signals applied to signal inputs into acoustic surface waves. At its integration electrode it is possible to pick off an output signal which corresponds to the convolution product of the electrical input signals. In order to compensate errors caused by the nonuniformity of the surface wave convolver in consequence of inhomogeneities of its integration electrode and of the piezoelectric crystal, a constant input signal is applied to one of the signal inputs and a predeterminable signal which determines a desired pulse response of the surface wave convolver, it is applied to the other signal input of the surface wave convolver. The output signals are freed from stochastic disturbing components by averaging to form the compensation quantity which is stored in a memory. The compensation quantity is used to compensate the output signal of the surface wave convolver convolving with the predeterminable signal in synchronism with the reading out of the compensation quantity from memory, when input signals to be processed are applied to the signal input.

BACKGROUND OF THE INVENTION

An article by Dr. techn. H.-P. Grasal in "Elektronik" 6/22.03.1985,pages 61 et seq., discloses a surface wave convolver which is used as asignal-processing component. The surface wave convolver describedtherein consists of an elongate piezoelectric substrate, to one end ofwhich a first interdigital transducer is fitted, one connection of whichserves as signal input for electrical input signals to be processed.These input signals are converted by the interdigital transducer intoacoustic surface waves which propagate along a propagation surfacedefined by an integration electrode on the piezoelectric substrate. Atthe other end of the piezoelectric substrate there is disposed a furtherinterdigital transducer, one connection of which serves as a signalinput for a predeterminable electrical signal which is converted, in asimilar manner, into acoustic surface waves which propagate on thepiezoelectric substrate in the direction of the first interdigitaltransducer. In this manner, the surface wave convolver executes aphysical folding of the electrical signals which are present at itssignal inputs. The signal output of the convolver is formed by a pointof connection of the integration electrode. At this signal output it ispossible to pick off a convolver output signal, which is proportional tothe convolution integral of the two input signals, as long as thosesignal components of the input signals which contribute to the result ofthe convolution are found, in a condition entirely converted intosurface waves, under the integration electrode of the surface waveconvolver. The integration period of the surface wave convolver isdetermined by the ratio of the length of the integration electrode tothe propagation velocity of the surface waves on the substrate.

An article by H.-P. Grassl and H. Engan ("Small-Aperture Focusing ChirpTransducers vs. Diffraction-Compensated Beam Compressors in Elastic SAWConvolvers" in "IEEE Transactions on Sonics and Ultrasonics", Vol.Su-32, No. 5, Sep. 1985) discloses that real surface wave convolversexhibit a nonuniformity. This is due, among other reasons, to the factthat the signal components forming the convolver output signal originatefrom arbitrary locations under the integration electrode. In order topermit an approximately equal amplitude weighting and phase retardationfor all these signal components on their path to the signal output, theintegration electrode is designed as a gathering and matching gridhaving a plurality of pick-off points uniformly distributed over itsentire length. A variation of the electrical parameters in the gridstructure of the integration electrode, a variation of the seriesresistance of the propagation surface and, especially, standing-waveeffects of nonlinearly generated longitudinal waves where the substratethicknesses are not constant lead to a nonuniformity of the surface waveconvolver. Accordingly, the uniformity is interpretable as a weightingfunction of the integration electrode. In operation of the surface waveconvolver, i.e., when the input signals to be processed are applied toone of its signal inputs and a predeterminable signal, which correspondsto the desired pulse response of the surface wave convolver(programmable filter), is applied to its other signal input, thenonuniformity leads to a situation in which that output signal of thesurface wave convolver which represents the result of convolution of theinput signals is affected by a considerable error. Even where thegreatest care is taken in the production of the surface wave convolver,the abovementioned causes of the nonuniformity cannot be entirelyeliminated.

SUMMARY OF THE INVENTION

The present invention provides a simple process by which the influenceof the nonuniformity of a surface wave convolver on the output signalcan be compensated.

According to the present invention, there is a process for obtaining acompensation quantity to compensate the nonuniformity of a surface waveconvolver that has a first signal input to which input signals to beprocessed can be applied, a second signal input to which apredeterminable signal can be applied and a signal output. A constantinput signal is applied to the first signal input and at the same timethe predeterminable signal is applied to the second signal input. Anoutput signal which can be picked off at the signal output is fed to amemory and is stored in the latter as a compensation quantity. The inputsignals to be processed are then applied to the first signal input whilemaintaining the predeterminable signal at the second input and thecompensation quantity is fed to the output signal of the surface waveconvolver in synchronism. In this manner, a compensation quantity isassociated with the predeterminable signal at the second signal input ofthe surface wave convolver in a simple manner, by which compensationquantity the error of the output signal of the surface wave convolverdue to the nonuniformity can be compensated for to an extremely greatextent. It can be shown, by considerations based on signal theory, thatthe remaining residual error of the output signal due to thenonuniformity is zero in the case of constant input signals to beprocessed, while in the case of non-constant input signals to beprocessed the error of the convolver output signal can be compensatedapart from a residual error which is determinable in accordance with theformula: ##EQU1## where the following symbols have the meaningsindicated: g_(x) (t): compensated output signal of the surface waveconvolver,

g_(u) ^(i) (t): output signal of a uniform (theoretical) surface waveconvolver,

u(t)f V(t): input signals of the surface wave convolver,

T_(o) : pulse duration of the predeterminable signal.

A particularly advantageous aspect of the process according to thepresent invention is that the error due to the nonuniformity of thesurface wave convolver can be reduced to zero without changes to thephysical structure of the surface wave convolver and without costlyexternal wiring arrangements. A further advantage is that the processaccording to the present invention can be performed automatically in asimple manner, for example in all cases where a differentpredeterminable signal is to be employed. Since a compensation quantityis definitely associated with each predeterminable signal, thiscompensation quantity can be filed in a memory and called up again whenrequired.

An advantageous further development of the process according to thepresent invention provides that the compensation quantity is invertedand the inverse compensation quantity is multiplied in synchronism bythe output signal of the surface wave convolver. The linkage of theoutput signal with the inverse compensation quantity represents a feedof the compensation quantity which can be performed in a particularlysimple manner in terms of circuit technology.

BRIEF DESCRIPTION OF THE DRAWINGS

The process according to the present invention is explained in thefollowing with reference to the drawings.

FIG. 1 shows a signal progression for recording a compensation quantity.

FIG. 2 shows a progression of the convolver output signal at theconvolver output according to FIG. 1.

FIGS. 3 to 9 show further signal progressions to explain the processaccording to the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a surface wave convolver C having two signal inputs E1 andE2. The signal input E1 has applied thereto a constant input signalu(t), which is normalized to the value 1. In this case, thenormalization to the value 1 means that the input signal u(t) permits amaximum control of the surface wave convolver C. The signal input E2 hasapplied thereto a predeterminable signal v(t) in the form of arectangular function with a pulse duration T_(o). The predeterminablesignal v(t) determines the pulse response of the surface wave convolverC which acts as a (programmable filter). The pulse response of a systemis understood to refer to the system reaction (i.e. the output signal)of the system when a Dirac pulse is applied on the input side (cf., forexample, Otto Mildenberger "Grundlagen der Systemtheorie furNachrichtentechniker" ("Principles of system theory for communicationsengineers"), Hanser Verlag, 1981, pp. 48-50). At an output A of the surface wave convolver C, it is possible to pick off an output signal

    c(t)=T.sub.o ·E(t)                                Equation 1

which represents the convolution product of the surface wave convolverC, effected by an error function E(t), and is proportional to the pulseduration T_(o) of the predeterminable signal v(t). However, themagnitude of the output signal c(t) can be increased only up to certainlimits by increasing the pulse duration T_(o), as the surface waveconvolver C can process only time-limited input signals. The recordingof the output signal c(t) is repeated n-times, and the respective outputsignals c₁ (t) to c_(n) (t) are fed to a memory component SP1 and storedin the latter. After this, an averaging of the output signals c₁ (t) toC_(n) (t) is undertaken in an averaging stage M. This leads to theformation of a compensation quantity K(t), which is free from stochasticdisturbing influences and which can be interrogated at the output of theaveraging stage M.

A typical progression of an error function E(t), which shows thenonuniformity of a surface wave convolver, is represented in FIG. 2. Itis possible to observe a relatively sharply defined progression of theerror function E(t), the duration of which is determined by the durationof integration T_(i) of the surface wave convolver. The error functionE(t) shown arises as a result of the folding of a rectangularpulse--which exhibits, in approximation to a Dirac pulse, a relativelyshort pulse duration T_(o) in the ns nanosecond range--aspredeterminable signal v(t) with a constant input signal u(t). In thecase of an entirely uniform surface wave convolver, there would be anexact rectangular pulse, the duration of which would correspond to theduration of integration T_(i) of the surface wave convolver. On theother hand, FIG. 2 shows periodically recurrent relative minima of theerror function E(t), which give an indication of the arrangement of thepick-off points of the integration electrode of the real surface waveconvolver.

FIG. 3 shows the surf ace wave convolver C in normal operation, in whichinput signals u(t) to be processed are applied to signal input E1 andthe predeterminable input signal v(t), which determines its pulseresponse, is applied to its signal input E2, which predeterminable inputsignal corresponds to the input signal v(t) according to FIGS. 1 and 2.The inverse compensation quantity K⁻¹ (t) is stored in a memorycomponent SP2, which inverse compensation quantity is formed byinversion of the compensation quantity K(t). A synchronising device Ssynchronises the inverse compensation quantity K⁻¹ (t) which can bepicked off from the memory component SP2, having regard to the transittime delays, occurring as a result of physical effects in the surfacewave convolver, with the time progression of the predeterminable inputsignal v(t). The inverse compensation quantity K⁻¹ (t) is multiplied, bymeans of a multiplier M, by the output signal c(t) of the surface waveconvolver C. This produces an output signal g_(k) (t), which is freefrom the error due to the nonuniformity of the surface wave convolver C,entirely (where the input signal u(t) to be processed is constant withina time interval of t±T_(o) /2) or to a large extent (if the input signalu(t) is not constant within the time interval t±T_(o) /2).

FIG. 4 shows the time progression of an input signal u(t), which isapplied to the signal input E1 of the surface wave convolver C accordingto FIG. 3. The output signal c(t) at the output A of the surface waveconvolver C exhibits a progression according to FIG. 5. In this case,the output signal c(t) is weighted with the pulse response of thesurface wave convolver C, which pulse response is predetermined by v(t);as a result of the nonuniformity of the surface wave convolver C, theoutput signal c(t) exhibits considerable distortions V in the region ofdiscontinuous changes to the input signal u(t); in the other regions,the output signal c(t) is represented in an undistorted condition, forthe sake of a simplified representation. The time compression of thesignal c(t) in comparison with the input signal u(t) to be processed, bythe factor two, is substantiated in that the input signals of thesurface wave convolver C which are converted into acoustic surface wavesmove together in the convolver in opposite directions.

FIG. 6 shows a theoretical output signal g_(u) ^(i) (t) of a uniformsurface wave convolver where the input signal u(t) to be processed,which is in accordance with FIG. 4, is applied thereto.

FIG. 7 shows a uniformity error F_(u) (t) which is obtained from thepercentage deviation of the output signal c(t) of the nonuniform surfacewave convolver C from the output signal g_(u) ^(i) (t) of a uniform(theoretical) surface wave convolver according to FIG. 6.

FIG. 8 shows the progression of the output signal g_(k) (t) (cf. FIG. 3)after performance of the compensation, and FIG. 9 shows the progressionof the percentage residual error F_(k%) (t) after performance of thecompensation of the nonuniformity. It is possible to observe a apercentage residual error F_(k%) (t) in consequence of the nonuniformityof the surface wave convolver C in comparison with the uniformity errorF_(u) (t) represented in FIG. 7 and it is being possible to observe areduction of the error caused by the nonuniformity by a factor of theorder of magnitude of 10 (cf. FIG. 7).

What is claimed is:
 1. A process for obtaining a compensation quantityand compensating for a nonuniformity of a surface wave convolver,comprising the steps of:applying a constant input signal to a firstsignal input of the convolver and at the same time, applying apredeterminable signal to a second signal input of the convolver;feeding an output signal, taken from a signal output of the convolver,to a memory in which an average output signal is stored as acompensation quantity; applying input signals to be processed to thefirst signal input of the convolver while maintaining thepredeterminable signal at the second signal input of the convolver, andcompensating for the output signal of the surface wave convolver usingthe compensation quantity read from the memory in synchronism withconvolution of the signals to be processed.
 2. The process according toclaim 1, wherein the step of compensating comprises the sub-stepsofinverting the compensation quantity, and multiplying the inversecompensation quantity, in synchronism, by the output signal of thesurface wave convolver.
 3. A process for obtaining an output signalusing a surface wave convolver, which signal is compensated with respectto a defect caused by the non-uniformity of the surface wave convolver,comprising the steps of:applying a constant input signal to a firstsignal input of the convolver and at the same time applying apredeterminable signal to a second signal input of the convolver,wherein constant input signals to be processed are applied to the firstsignal input while maintaining the predeterminable signal at the secondsignal input; performing a convolution integral of the constant inputsignal and of the predeterminable signal and providing a signalproportional to the convolution integral as an output of the convolver;providing the output signal of the convolver to a memory and storing anaverage output signal in the memory as a compensation quantity; andmultiplying the compensation quantity in synchronism with the outputsignal of the surface wave convolver to obtain a compensated outputsignal.